1. Field of the Invention
The invention is in the field of pyrolysis in tubular furnaces.
2. Description of the Prior Art
Light aromatic hydrocarbons, comprising benzene, toluene, the xylenes, ethylbenzene and styrene, are presently mainly derived from petroleum oil and are the major building blocks for many of the commercially important synthetic resins and plastics. Many of these are also relatively crucial in supplying the octane enhancement necessary in the higher performance gasolines, especially the increasingly important lead-free higher octane gasolines.
In coming years, however, the world supply of the lighter crude oils from which economical yields of these aromatics can be obtained will be diminishing. While the world supply of total crude petroleum oil is expected to increase very slightly (1% per year) over the next one or two decades, the desirable light crude is expected to gradually diminish as it is and has been preferentially utilized.
Meanwhile, relatively abundant amounts of natural gas are being found, especially in places remote from markets. Worldwide natural gas is expected to increase in usage by about 2.2% per annum, and the probable and potential reserves thereof are expected to rival the sum of those of oil shale and tar sands combined. The concomitant supply of the lighter portion of the natural gas liquid fraction--comprising ethane, propane and the butanes--is expected to be in excess supply at some of the more remote locations. This so-called liquified petroleum gas (LPG) is relatively difficult to transport from remote locations, and so it is a candidate for conversion to more easily transportable and more valuable products.
Commercially, LPG is a well-known and widely used raw material for the production of such olefins as ethylene and propylene by pyrolytic means. However, these products are even more difficult and costly to transport than their precursors.
Invariably, in the well-known production of ethylene and propylene from aliphatic hydrocarbons by pyrolysis some aromatic by-products are produced. However, especially with LPG as the precursor raw material, the yield is often so small that, rather than being a fruitful source of these compounds, their production is in nuisance proportions, to be separated from the desired product and quickly disposed of--for example, as furnace fuel.
Much research and development effort has been expended upon pyrolytic means, including much directed towards enhancing the yield of valuable aromatic compounds. It has, for example, been taught by Smith and Boston (U.S. Pat. No. 2,852,440, issued Sept. 16, 1958) under the auspices of Esso Research and Engineering, that by pyrolysis in a first zone at relatively high hydrocarbon partial pressure, e.g., 30-100 psia, and in a second zone at relatively low partial pressure, e.g., 2-20 psia, a higher yield of aromatics is obtained, along with a relatively high yield of unsaturated compounds. However, such a high pressure drop and low offgas partial pressure entails very costly subsequent compression with very large and expensive compressors or very large amounts of increasingly costly dilution steam, and usually both.
Furthermore, the bulk of the aromatic content of the pyrolytic offgas arises in the first relatively high pressure zone, and must pass through the second zone where the bulk of the desired unsaturated products are produced. Aromatics are thus present in relatively high (i.e., as high or higher than product concentrations) in the presence of unsaturates at their full product concentrations. Under such pyrolytic conditions much alkylation of the desired aromatics by the unsaturates will rapidly take place, thus producing relatively large quantities of relatively useless aromatic tar, rather than the most desired lower members, benzene, toluene and xylenes (so-called BTX).
Still further, in order to obtain BT yields of only about 13 weight percent, an inordinately long residence time is required in the first zone, i.e., about three seconds. Such a long residence time would require either inordinately long tubes, or low, inefficient-for-heat-transfer velocities or both.
Even further, Smith and Boston do not contemplate producing aromatics from LPG, but rather from gas oil (preferably boiling between 430.degree. F. and 1000.degree. F.).
It has been long known, of course, that higher molecular weight products, much of it in the gasoline boiling range, can be produced from unsaturated gases. This product has been known as "poly-gasoline", and the typical process used was generally described as in 1935 by C. R. Wagner (Industrial and Engineering Chemistry, Vol. 27, pages 933-936), in which in a commercial unit as much as 25 gallons of such product per 1000 cubic feet of gas were obtained by recycling unsaturates through a zone at 950.degree. F. and 800 psig. This product was of only moderate octane number (ASTM method, 78), indicating only a modest aromatic content.
In parallel research efforts cited by Wagner, however, it was also found that, if the "olefin-bearing gases were heated quickly to 1100.degree. F. or higher and then allowed to rise in temperature because of the exothermic heat of reaction until a final temperature of 1200.degree. F. to 1300.degree. F. was reached, . . . a highly aromatic distillate from which gasoline having an octane number (ASTM) of approximately 100 could be produced." However, no other details of the conditions were given, except a mention (in the abstract) that the pressure was "low", presumably relative to the above-cited 800 psig.
In later experimental work, such as that of Towell and Martin (AICHE Journal, Vol. 1, pages 693-8) reported in Dec., 1961, it was found that in pyrolyzing ethylene at temperatures between about 600.degree. C. and 1200.degree. C. that "hydrogen, 1, 3 butadiene and acetylene were considered to be the major primary products", and that the presence of propylene greatly inhibited the pyrolysis of ethane to ethylene (their FIG. 10).
Fairly recently, in 1969, Kunugi, et al. (I & EC Fundamentals, Vol. 8, pages 374-383) reported a comprehensive study of the thermal reaction of ethylene at temperatures between about 703.degree. C. and 854.degree. C., at initial ethylene partial pressures between about 0.25 and 1 ata (atmospheres absolute), at various residence times between about 0.25 and 2.4 seconds, and with additives including ethane and butadiene. The same laboratory had previously similarly studied the thermal reaction of propylene and butadiene in papers presented to the Chemical Society of Japan and the Japan Petroleum Institute, respectively. Kunugi, et al. concluded that ethane initiated the reactions and that butadiene was a necessary intermediate in the formation of aromatics. However, with either of these additives in the best of their reported results, the yield of BT was only 0.787 and 0.327 (their tables III and IV), mols per 100 mols of feed respectively. Selectivity to benzene was less than about 8.5 mols per 100 mols of ethylene converted.
The same laboratory then reported a comprehensive study (Kunugi, et al., I & EC Fundamentals, Vol. 9, pages 314-324), in 1970, upon the thermal reaction of propylene. Here Kunugi, et al., concluded that the initiator was 1-butene and that the selectivities to the main products are independent of temperatures from 750.degree. C.-850.degree. C. Their results show selectivities to benzene up to about 6 mols per 100 mols propylene converted and less than 3 mols/100 converted for toluene.
Finally, under the auspices of Socony-Vacuum (now Mobil), Kinney and Crowley (Industrial and Engineering Chemistry, Vol. 46, pages 258-64) reported in 1954, a comprehensive laboratory study aimed at maximizing the production of aromatics from refinery gases (including some ethylene and propylene along with the LPG gases), as shown in their Table II. Yield of light aromatics (C.sub.6 -C.sub.8) reported was less than 10.5 weight %. Furthermore, the yield of coke was always appreciable, and in some cases exceeded the yield of these desired aromatics. Such an amount of coke--even at the lowest of coke yields, 2.5 weight %--would be much more than enough to preclude the use of tubular pyrolysis furnaces, since the tubes would very quickly coke up, plug up, and at the process temperatures involved (1500.degree. F.), overheat and burn out rapidly in a fired tubular furnace.
Kinney and Crowley also investigated the yields of light aromatics from the individual gases: ethylene, ethane, propylene and propane, under conditions of maximum aromatic formation. Temperature was 1500.degree. F., residence times were within the range of 3-20 seconds, and the hydrocarbon partial pressure was (presumably) one atmosphere. Results are shown in their Table III, from which selectivities to light aromatics may be calculated as 25.2, 11.5, 14.9 and 10.8 weight percent, respectively. In every case the selectivity to heavy aromatic "tar" exceeded that to the desirable light aromatics, and in some cases approached twice the selectivity to the light aromatics. Their FIG. 8 shows the selectivity to coke, which at these conditions varied from about 2.5 to about 9 weight percent.
The above art is by no means exhaustive, but is representative of the better attempts, as well as of the practical difficulties involved in attempting to commercially utilize tubular furnaces to produce BTX from LPG and the like in relatively high ultimate yields in an economical process. To the present authors' knowledge, there has been no disclosure of a practical means or conditions of so doing. In view of the clear economic incentives both as cited hereinabove and as implied by the persistence over the years represented by the work cited above signifying the numerous attempts to obtain such a practical means, it is clear that discovering and devising such a means is well beyond mere ordinary skill in the art.